Giant absorption of light by molecular vibrations on a chip
A. Karabchevsky,1,* and A. V. Kavokin,2,3‡
1
2
Department of Electrooptic Engineering, Ben-Gurion University, 84105, IL
Department of Physics and Astronomy, University of Southampton, SO17 1BJ, UK
3
CNR-SPIN, Viale del Politecnico 1, I-00133 Rome, Italy
Vibrational overtone spectroscopy of molecules is a powerful tool for drawing
information on molecular structure and dynamics1. It relies on absorption of near
infrared radiation (NIR) by molecular vibrations. Here we show the experimental
evidence of giant enhancement of the absorption of light in solutions of organic
molecules due to the switch from ballistic to diffusive propagation of light 2-6 through a
channel silicate glass waveguide. We also experimentally address a dynamics of
absorption as a function of time of adsorption of the organic molecules on a waveguide.
The observed enhancement in diffusion regime is by a factor of 300 in N-Methylaniline
and by factor of 80 in Aniline compared to the expected values in the ballistic
propagation of light in a waveguide. Our results underscore the importance of a guide
surface modification and the disordered6 molecular nano-layer in enhancement of
absorption by amines on engineered integrated system7.
Aromatic amines, such as N-Methylaniline (NMA) for which Aniline is a prototypical
molecule, are widely used in biology and medicinal chemistry. In particular, they constitute a
crucial component of many drugs, pesticides and explosives. Since most amines are basic,
salts can be generated to facilitate water solubility or solubility in other vehicles for drug
administration8. Historically, the amines were also largely employed in conjugating aromatic
rings for applications in the dye industry.
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Nowadays, the interest in the vibrational spectra of amines has been boosted by the
potential application of molecular vibration spectroscopies in the detection of explosive
materials and diagnostics of psychoactive stimulants based on amines9. The most wide-spread
method of vibration spectroscopy of molecules relies on the operations in a mid-infra-red
(Mid-IR) spectral range. Its main disadvantages are the needs for bulky and expensive
equipment and for significant quantities of sample solutions in order to extract high precision
data on spectral positions of vibrational modes. From the perspective of point of care
applications, there is a need for a micro-size molecular spectroscopy set-up suitable for
operations with low volume samples. Here we demonstrate a near-infra-red (NIR)
spectroscopy laboratory on a chip capable of high precision measurements of resonant NIR
absorption spectra of organic molecules10. We have performed the transmission spectroscopy
measurements on samples containing Aniline and NMA molecules. The absorption efficiency
of the solution placed in a close vicinity to a silicate glass channel waveguide has been found
to be increased by a factor of up to 300 compared to the theoretical value obtained assuming
the ballistic propagation of NIR in the waveguide channel. We interpret this dramatic
enhancement in terms of the switching between ballistic and diffusive propagation of light11
induced by the resonant scattering of light by organic molecules.
Figure 1a shows an overtone vibrational spectrum of the pure NMA and of the NMA /
hexane (Hex) mixtures photographed in Figure 1e. The inset (Figure 1b) shows the
absorption bands of NMA due to the (N-H)-bond-stretching band around 1.5 m and the aryl
(C-H) overtone band around 1.65 m in an overtone region V = 2. The skeletal molecular
structure of N-Methylaniline is shown in Figure 1c.
2
Figure 1| An overtone vibrational spectrum of the organic molecule N-Methylaniline. a, Transmittance spectra of pure NMethylaniline (NMA), pure hexane (Hex, black curves) and NMA diluted in Hex measured by Jasco V570
o
spectrophotometer recorded at room temperature of 21±2 C and converted to dB units. The inset b shows the absorption
bands of NMA due to the (N-H)-bond stretching band around 1.5 m and the aryl (C-H) overtone band around 1.65 m in
the V = 2 region. Shift in location of the N-H band with respect to increase in concentration indicated by the dashed line.
Arrows are pointing to a change in the signal strength and width. Skeletal molecular shape of NMA is shown in the inset c.
The quartz cuvette pathlength is 5 mm and the measurement was performed at normal incidence as drawn in d with air as
reference. e, Photograph of the pure NMA and diluted mixtures prepared for the measurement with colored concentration
values corresponding to the colors of the spectral curves.
We emphasize that although broadband illumination probes many electronic states, our
waveguide, optimised for the single mode regime around 1.5 m, only supports overtone
electronic states in the 1st overtone region (V=2) corresponding to the transition from the
ground molecular state to the second size quantized energy state. Note that in the vicinity of
the 1rst and second overtones the molecular potential acting upon the electronic excitation is
nearly parabolic (see Figure 1f).
Integrated optics or ‘thin film guided wave optics' is harnessing from miniaturization
due to the manipulation of the optical waves guided in a thin film deposited on a lower
refractive index substrate12. Light can be guided in a doped substrate if the dopant induces an
increase of its refractive index. Here we used diffused K+ ions in the silicate glass to form the
guiding layer with a higher index. The optical mode in such structures can be also confined
laterally due to the dopant concentration variation in the plane of the structure, see equation
(1). Therefore such a structure prevents lateral diffraction of NIR to sides of the guided
3
region. In diffusion based waveguides, lateral confinement is governed by the width of the
dopant distribution as in equation (1). Light guiding within a thin film illuminated by the
monomode fiber13 enables transmittance spectroscopy studies of the media surrounding the
wave-guide due to the evanescent tails of the propagating light mode. Here we use fibrecompatible ion-exchanged waveguide in silicate glass operating in the monomode regime at
the wavelengths of the 1st overtone region of anilines (V=2). Figure 2b shows a photograph
of the waveguide with a polydimethylsiloxane (PDMS) liquid reservoir (see Methods section
for fabrication details) with input/output fibres guiding green and red laser lights for
demonstration of potential multi-array detection. During the experiment, a 0.1 mL droplet of
66.7% NMA in Hex mixture has been dripped onto the liquid reservoir with a micropipette
for spectral analysis. Figure 2c shows the NIR transmittance spectra of the waveguide which
is in contact with the solution of NMA molecules recorded using the set up shown in Figure
2a after about 30 min from the first contact of the solution with the structure. The signals
were recorded every 1 min. An immediate drop in transmittance has been observed using our
device after the first contact with the mixture. Figure 2c shows the time evolution of the
relevant transmittance spectra.
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Figure 2| Experimental set-up and recorded spectra of NMA adsorbed to the waveguide. a, Artist’s impression of the
experimental set-up based on butt-coupled broad-band illumination of a waveguide system by a supercontinuum NIR
source and recording of the transmittance spectra with use of OSA (See Methods). Tear-sized droplet of a liquid has been
dripped onto the waveguide for the spectroscopy analysis. b, photograph of the chip during the measurement, NMA has
been dripped in Hex solution onto PDMS liquid reservoir using micropipette; in the photograph, two waveguides are
illuminated by coherent red and green light sources for demonstration of potential multi-array detection. c, Transmittance
spectra (solid curves) recorded using the waveguide while dripping volume of 0.1 mL and concentration of 66.7 % NMA
diluted in Hex mixture. The spectra are registered at time intervals of about 1 min with time increasing from the top to the
bottom of the figure. The blue solid curve corresponds to the min 0 in the subplot (d). The red curve corresponds to the
transmittance decrease by 18 dB. Other curves have been shifted vertically for clarity. N-H and aryl C-H absorption bands
areindicated by the arrows; d, Experimental transmittance modulation depth of (N-H)-bond stretching band shown as a
function of time.
The effective concentration of NMA in the vicinity of the waveguide surface
increases with time and eventually achieves the maximum loss of about -18 dB (Figure 2d)
which corresponds to the loss measured for the pure NMA sample in a cuvette (Figure 1b).
NIR measurements from cuvettes of different NMA/Hex mixtures illustrate that the presence
of Hex in the mixture shifts the N-H peak toward shorter wavelengths (Figure 1a-b and
Figure 2c). NIR data recorded on a waveguide from the droplet of 67% NMA diluted in Hex
shows that in 35 minutes the centre of the N-H peak shifts from 14.75 m to 14.9 m,
indicating that the concentration of NMA within the penetration depth of the evanescent NIR
field increases with time. This signal then remains steady, demonstrating that a compact
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organic layer has formed near the surface and the Hex has been pushed beyond the
evanescent field region (Figure 2d). Complementary surface analytical techniques were used
to characterize the formed layer: ellipsometry and contact-angle measurements (see
Supplementary Section 3.3).
Let us emphasize the magnitude of the observed effect. Channel waveguides were
designed for the monomode operation and high sensitivity at a wavelength of 1.5 m using
the finite element method (FEM) and employing an approximate diffusion profile of
potassium ions in glass14 (Figure 3c) according to equation (1) with the maximum core index
of 1.5105.
𝑦
𝑛(𝑥, 𝑦) = 𝑛𝑠 + ∆𝑛𝑒𝑟𝑓𝑐 (𝑑𝑦) 𝑒 −𝑥
2 ⁄𝑑𝑥 2
(1)
With substrate refractive index of ns=1.5013 at 1.496 m. The coordinate y in equation (1) is
the distance from the surface and x is the lateral distance from the waveguide symmetry axis;
n is the maximum increase of the refractive index due to the dopant, which is 0.0092 for
TM- and 0.008 for TE-polarised modes, dx is the profile depth and dy is the profile half-width.
Figure 3b shows the calculated transmittance of our device, in ballistic regime, in the
wavelengths range of N-H stretching band of NMA in region V=2. The resonant scattering
leads to switching from the regime of ballistic to diffusive propagation of light2,3 - a
phenomenon which is well known e.g. in most biological tissues, where photon propagation
is quickly randomized due to the elastic scattering. This results in a background loss in
optical transmittance and a strong delay in the transmission of light around the resonance4. It
is important to underline that in the diffusion regime photons propagating through a
waveguide are absorbed with a higher probability than in the ballistic regime, in general. This
tendency has been recently established by the experiments on exciton-polaritons propagating
through crystal slabs11. Qualitatively, the longer time a photon spends in an absorbing
medium the higher chances it has to be absorbed.
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Figure 3| Transition from ballistic to diffusive ‘Slow light’ propagation in a guide due to the diffusion by NMA molecules.
a, Measured absorbance of a pure NMA and the corresponding calculated ; b, Calculated transmittance in the ballistic
regime with pure NMA; c, Calculated refractive index profile n(x,y) for TE polarization of the waveguide using COMSOL
Multiphysics 4.3b, photon ballistic pathlength L and photon elastic scattering mean-trajectory 𝐿̃ are shown schematically; d,
Schematic showing the ballistic (yellow arrows) and diffusive (red arrows) pathways of NIR photons in the waveguide
(substrate region is not shown for simplicity) with organic molecules adsorbed to the surface. The blue curve describes the
electric field intensity spatial distribution of the fundamental mode in a guide |ℇ𝑦 (𝑥, 𝑦, 𝑧)| .
We have estimated the absorption amplification due to the diffusive propagation of
light through the waveguide. The extinction coefficient 𝜅(𝜆):
𝜆
𝜅(𝜆) = 𝑙𝑜𝑔𝑒 (10𝐴(𝜆) ) 4𝜋𝐿
(2)
has been deduced based on the Beer-Lambert low and measured absorption (A) data using a
spectrophotometer with 10 mm path-length (Figure 3a). Transmitted power through the guide
was calculated as 𝑃 ≈-0.06 dB for ballistic propagation of light through the guide (Figure 4b)
with L≈ 3 mm where:
𝑃 = −10𝑙𝑜𝑔(𝑒 −4𝜋𝐿𝜅⁄𝜆 )
(3)
Here L is the length of the contact region of the waveguide with the liquid. The switch to the
diffusive propagation, results in the replacement of L by 𝐿̃ >> L. In this case, the outgoing
signal is reduced by a factor of 𝐿̃/L= (P’out/Pout), where P’out is a measured power, 𝐿̃ is the
estimated mean trajectory of a photon in the diffusive medium, and P’out is a transmitted
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power calculated using L as the trajectory length in the ballistic regime. Taking =9.4314x105
, m, P’out=-18 dB, Pout=-0.06 dB, we obtain 𝐿̃ ≅ 0.9 m. This differs from the
theoretical expectation for the ballistic propagation case by a factor of about 300. We explain
this giant discrepancy by two crucial effects: (1) the chemical adsorption of organic
molecules to the surface of the guide, their reorganisation and the consequent formation of a
dense structured layer; and (2) the physical effect of resonant scattering of NIR by disordered
film of organic molecules. In the modelling we performed using the Maxwell equation solver,
the increase in the effective thickness of adsorbed molecular layer as a function of time was
insufficient to explain the giant modulation of absorption within the ballistic model. We
argue that the resonant scattering of light by organic molecules placed in the close vicinity to
the waveguide is responsible for the observed effect. The scattering causes the photon
propagation direction to change randomly as shown schematically in Figures 3c-d. The
Maxwell solver is no more convenient for description of light propagation in this regime.
Instead, a diffusion equation for photons should be used (see e.g. equation (4) in Ref. 4 and
Supplementary Information Section 1-2).
An increased absorption and delay of light near the resonance was observed both in
GaN and in ZnO materialShubina1, Shubina2. Here, the diffusive regime is established because of
the adsorption of organic molecules on the surface of the waveguide (see Supplementary
Information Section 3). The formation of an 8 nm thick organic layer has been detected by
the ellipsometry. Attraction of NMA (index of nNMA=1.571) to the modified surface of our
device is driven by the surface tension between the non-polar solvent, Hex, and the polar
waveguide surface. The polar N-H bond in NMA can align with the polar surface, and the
non-polar benzene ring aligns with the solvent, relieving the surface tension (Figure 4a).
Since benzene rings also tend to stack together, and N-H bonds in different NMA molecules
will form hydrogen bonds with each other, a multilayer structure can form much like a
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lamellar liquid crystal15. Contact angle measurements shown in Figure 5b confirm the
waveguide surface modification due to adsorption. In addition to the aromatic ring, NMA
molecules contain a methyl group such as in Toluene (index of nToluene=1.4968) and an amine
group such as in Aniline (index of nAnl=1.5863). The spectral responses of those molecules
on our device are shown in Figure 4c. However, despite the fact that the N-H overtone
absorption band of Aniline in the NIR spectrum has stronger absorption compared to NMA, it
has a less pronounced transmittance resonance on a guide. This observation can be explained
by the increased hydrogen bonding existing between the amine molecules in aniline, which
makes it less energetically favourable for them to reorganise and align with the surface
(Figure 4d)16. The N-H overtone bands may be shifted, as in Aniline, by changing the
solvents and temperature conditions. The effects of hydrogen bonding on the mechanical
anharmonicity of the N-H stretching vibration have been reported17,18. Weak hydrogen
bonding appears to reduce the anharmonicity of the vibration resulting in the broadening of
the overtone absorption peak width17,19.
Figure 4| Responses from benzene molecules with different substituents demonstrating the influence of hydrogen
bonding. a, schematic illustrating a multilayer structure of NMA formed on the surface of the waveguide in the 67%
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NMA/Hex mixture. Dashed lines represent hydrogen bonding between two moieties; b, surface modification measurement
of planar waveguide with the contact angle ∠ of the water droplet on the clean glass as ∠10°, ∠20° close to the
immersion area strongly exposed to the solution vapour and a droplet of water on the immersed surface with a contact
angle of ∠56°; c, NIR transmittance spectra of Toluene, aniline and NMA on our device; d, molecules used in this study
ordered according to the number of hydrogen bonds they can form with other moieties (Toluene: 0, NMA:1 and Aniline:2)
To conclude, due to the chemical adsorption process, a diffusive layer rich with
organic molecules settled off the solution has been formed at the surface of the waveguide.
This layer is responsible for the switch from ballistic to diffusive propagation of NIR
radiation in the waveguide and the giant absorption detected. Compared to the demonstrated
detection of N-Methylaniline on planar integrated optical component20,21, a ring resonator22 or
microtapered fiber20 our system achieved dramatically stronger absorption signal of N-H
overtone at around 1.5 mm wavelength that harnesses cheap waveguide fabrication procedure
optimised for NIR (telecommunication) wavelengths. Detection of the primary amine,
Aniline on a waveguide was shown for the first time. The affinity of aromatic amines to the
surface of an integrated optical device makes our approach particularly suitable for NIR
detection and spectroscopy studies of amine based group materials such as anilines important
for pharmaceutical and other chemical industries23.
Methods
Sample fabrication and characterization.
The waveguides were fabricated by photolithographical patterning of 250 nm thick hard mask on
silicate glass. This results in the stripe openings of 6 m width. Then, the glass was immersed in a
KNO3 molten for 11 hr at 395°C. After ion-exchange the end facets of the glass were polished
perpendicular to the waveguide resulting in formation of waveguides length ~35 mm. Planar
waveguides were characterised using Metricon prism coupler and found to be single mode operating
at 1.5 m and having three modes at 632.8 nm. Effective index at 632.8 nm using transverse electric
(TE) polarization of the fundamental mode (zero order mode) is 1.5191, 1.5170 for the first order
mode and 1.5157 for the second order mode. At 1.5 m effective index of the fundamental mode was
measured as 1.5093. In addition, channel ion-exchanged waveguides were characterised by imaging
the mode profile at the output of the waveguide on IR camera through the x10 focusing length and
found to be single mode at 1.5 m and having three modes at 632.8 nm. Note, since the waveguides
are optimised for telecommunication wavelengths, they exhibit surface scattering loss for visible light.
This is why they can be visualised with green and red lasers as photographed in Figure 2b. The near
field mode profiles were measured to estimate dimensions of the mode profile from our device
compared to the mode of SM1550 fibre at the same distance from IEEE-1394 Digital Camera using
x10 objective and neutral density (ND20) filter to protect the camera from saturation. Mode profile of
SM1550 fibre is about 9.3 m. The width and depth of the mode profile of our device were found as
5.2565 m and 7.6826 m respectively.
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Solvent cleaning.
Waveguides have been cleaned following standard solvent cleaning routine while increasing solvent
polarity. Waveguides have been sonicated for 20 min ultrasonic bath, and then rinsed with
isopropanol then with deionised water and blow dried with air gun.
Surface modification.
Waveguides were cleaned from organic residues following solvent cleaning routine (see above) and
negatively charged using Tepla 300 Plasma Asher machine at 1000 W with gas O2 of 600 ml/min until
the temperature of 145°C was reached. The process took about 5-7 min. The power then was turned
off to cool down the waveguides to 55°C. The procedure has been repeated once more in order to
generate higher oxygen anions charge on the surface of the waveguides. This method yields a very
hydrophilic surface.
Fabrication of the liquid reservoir.
To conduct the measurements of liquid solutions on waveguides, the polydimethylsiloxane (PDMS)
liquid reservoir has been fabricated. The Sylgard 184 elastomer and curing agent were mixed together
at a weight ratio of 10:1. Liquid PDMS has been placed in vacuum conditions for 30 min, and then
baked at 90°C for 1 hr, resulting in a robust chamber. Bath-like shape has been formed using
aluminium foil supported by a microscope glass. By inspecting the spectrum of the PDMS we
confirmed that it is a non-absorbing material at the wavelengths of interest. A demountable PDMS
chamber of length 8 mm was self-stick on top of the waveguides to contain the liquid. After 45 min of
measurements, PDMS chamber was detached from the waveguide and the droplet of ~3 mm in
diameter remained on the surface.
Optical spectroscopy on a waveguide.
Optical waveguide measurements of transmittance spectra were conducted using the apparatus shown
in Figure 2a. The measurements were realised using the infra-red light generated by a high power
fibre continuum source (Fianium SC-600-FC) which is operating at the central wavelength of 1060
nm with spectral bandwidth spanning from 450 nm to beyond 1750 nm and generating optical pulses
with duration less than 10 ps. Broadband light was collimated and focused onto polarisation
maintaining fibre (PMF) using Melles Grot objective NA 0.65 and magnification x40 to control
polarization in the system. PMF was directly fibre-coupled into a channel waveguide (with coupling
efficiency of 8 dB/nm) and the power transmitted through the waveguide was fibre-coupled using
graded index multimode fibre into an optical spectrum analyser (OSA, Yokogawa AQ6370). Spectral
resolution was set to 0.5 nm. The acquisition time for the 300 nm spectral window is about 50 sec. No
polarization dependency was confirmed. NMA (index of nNMA=1.57118) was diluted in Hex (index of
nHex=1.37508) ratio 2:3 resulting in the mixture index of 1.46769. Note: indices of the liquids were
measured using RA 510 refractometer operating at 589 nm at room temperature of 21±2oC. At the
beginning of the measurement, a droplet of NMA in Hex volume 0.1 mL has been dripped onto a
PDMS liquid reservoir placed on a waveguide for the chemical analysis. Demonstration of the
dynamics of NMA on a guide lasted for 42 min. To conduct the measurements with Aniline, prior the
measurement, the waveguides were cleaned and charged by oxygen anions in Tepla 300 Plasma Asher
machine.
‡
Electronic address: *[email protected]; [email protected]
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Acknowledgments
A.V.K. acknowledges support from the EPSRC established career fellowship and A.K. acknowledges
support from the BGU (Israel) an Outstanding Woman in Science Award. We thank J. J. Greffet, E.
Plum and N. I. Zheludev for helpful discussions.
Author contributions
A.K. proposed the idea of the paper, performed sample fabrication, characterised the samples, built
experimental setup, performed measurements and theoretical calculations. A.V.K. proposed the
interpretation of the results and developed the theoretical aspects. A.K. and A.V.K. analysed the
results and co-wrote the paper.
Competing financial interests
The authors declare no competing financial interests.
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